Digital data, which comprises a series of rectangular pulses, cannot be directly transmitted by means of voice frequency telephone lines because of the limited bandwidth of such lines. Digital data is normally transmitted via telephone lines by modulating a carrier frequency in the voice band and then transmitting the modulated carrier over the telephone line. Digital data is sometimes filtered to limit the bandwidth thereof and the filtered data used to modulate the carrier. Alternatively, the data may be used to directly modulate a carrier, the modulated carrier then being filtered before application to the telephone line. Prior art modulation techniques include frequency modulation, double sideband amplitude modulation, single sideband amplitude modulation, multi-phase modulation, and phase and amplitude (double sideband) modulation.